In recent years, with an aim toward effective utilization of energy for greater environmental conservation and reduced usage of resources, a great deal of attention is being directed to power smoothing systems for wind power generation or overnight charging electric power storage systems, household dispersive power storage systems based on solar power generation technology, and power storage systems for electric vehicles and the like.
The number one requirement for cells used in such power storage systems is high energy density. The development of lithium ion batteries is therefore advancing at a rapid pace, as an effective strategy for cells with high energy density that can meet this requirement.
The second requirement is a high output characteristic. A high power discharge characteristic is required for a power storage system during acceleration in, for example, a combination of a high efficiency engine and a power storage system (such as in a hybrid electric vehicle), or a combination of a fuel cell and a power storage system (such as in a fuel cell electric vehicle).
Electrical double layer capacitors and nickel-metal hydride batteries are currently under development as high output power storage devices.
Electrical double layer capacitors that employ activated carbon in the electrodes have output characteristics of about 0.5 to 1 kW/L. Such electrical double layer capacitors have high durability (cycle characteristics and high-temperature storage characteristics), and have been considered optimal devices in fields where the high output mentioned above is required. However, their energy densities are no more than about 1 to 5 Wh/L. A need therefore exists for even higher energy density.
On the other hand, nickel-metal hydride batteries employed in existing hybrid electric vehicles exhibit high output equivalent to electrical double layer capacitors, and have energy densities of about 160 Wh/L. Still, research is being actively pursued toward further increasing their energy density and output, and increasing their durability (especially stability at high temperatures).
Research is also advancing toward increased outputs for lithium ion batteries as well. For example, lithium ion batteries are being developed that yield high output exceeding 3 kW/L at 50% depth of discharge (a value representing the state of the percentage of discharge of the service capacity of a power storage element). However, the energy density is 100 Wh/L or less, and the design is such that high energy density, as the major feature of a lithium ion battery, is reduced. Also, the durability (especially cycle characteristic and high-temperature storage characteristic) is inferior to that of an electrical double layer capacitor. In order to provide practical durability, therefore, these are used with a depth of discharge in a narrower range than 0 to 100%. Because the usable capacity is even lower, research is actively being pursued toward further increasing durability.
There is a strong demand for implementation of power storage elements exhibiting high energy density, high output characteristics and durability, as mentioned above. Nevertheless, the existing power storage elements mentioned above have their advantages and disadvantages. New power storage elements are therefore desired that can meet these technical requirements. Promising candidates are power storage elements known as lithium ion capacitors, which are being actively developed in recent years.
A lithium ion capacitor is a type of power storage element using a nonaqueous electrolytic solution comprising a lithium salt (hereunder also referred to as “nonaqueous lithium power storage element”), wherein charge/discharge is accomplished by non-Faraday reaction by adsorption and desorption of anions similar to an electrical double layer capacitor at about 3 V or higher, at the positive electrode, and Faraday reaction by intercalation and release of lithium ions similar to a lithium ion battery, at the negative electrode.
To summarize the electrode materials commonly used in power storage elements, and their characteristics: when charge/discharge is carried out using a material such as activated carbon as an electrode, by adsorption and desorption of ions on the activated carbon surface (non-Faraday reaction), it is possible to obtain high output and high durability, but with lower energy density (for example, one-fold). On the other hand, when charge/discharge is carried out by Faraday reaction using an oxide or carbon material as the electrode, the energy density is higher (for example, 10-fold that of non-Faraday reaction using activated carbon), but then durability and output characteristic become issues.
Electrical double layer capacitors that combine these electrode materials employ activated carbon as the positive electrode and negative electrode (energy density: one-fold), and carry out charge/discharge by non-Faraday reaction at both the positive and negative electrodes, and are characterized by having high output and high durability, but also low energy density (positive electrode: one-fold×negative electrode: one-fold=1).
Lithium ion secondary batteries use a lithium transition metal oxide (energy density: 10-fold) for the positive electrode and a carbon material (energy density: 10-fold) for the negative electrode, carrying out charge/discharge by Faraday reaction at both the positive and negative electrodes, but while their energy density is high (positive electrode: 10-fold×negative electrode: 10-fold=100), they have issues in terms of output characteristic and durability. In addition, the depth of discharge must be restricted in order to satisfy the high durability required for hybrid electric vehicles, and with lithium ion secondary batteries only 10 to 50% of the energy can be utilized.
A lithium ion capacitor is a new type of asymmetric capacitor that employs activated carbon (energy density: 1×) for the positive electrode and a carbon material (energy density: 10-fold) for the negative electrode, and it is characterized by carrying out charge/discharge by non-Faraday reaction at the positive electrode and Faraday reaction at the negative electrode, and thus having the characteristics of both an electrical double layer capacitor and a lithium ion secondary battery. It also exhibits high output and high durability, while also having high energy density (positive electrode: one-fold×negative electrode: 10-fold=10) and requiring no restrictions on depth of discharge as with a lithium ion secondary battery.
In PTL 1 there is proposed a lithium ion secondary battery using a positive electrode containing lithium carbonate in the positive electrode, and having a current shielding mechanism that operates in response to increased internal pressure in the battery. In PTL 2 there is proposed a lithium ion secondary battery employing a lithium complex oxide such as lithium manganate as the positive electrode, and with reduced elution of manganese by including lithium carbonate in the positive electrode. In PTL 3 there is proposed a method of causing restoration of the capacitance of a deteriorated power storage element by oxidizing different lithium compounds as coated oxides at the positive electrode.
These methods, however, are associated with increased resistance due to inhibition of electron conduction between the active material particles by addition of lithium compounds, and reduced energy density, and therefore there has still been room for improvement in terms of high output and high energy density. Moreover, the change in potential of the positive electrode active material layer coated on the front and back sides of nonporous positive electrode power collector is not taken into consideration, and therefore there has still been room for improvement in terms of minimizing excessive decomposition of the lithium compound in the positive electrode active material layer.
PTL 4 proposes a power storage element employing activated carbon as the positive electrode active material, and as the negative electrode active material, a carbonaceous material obtained by intercalating lithium by a chemical process or electrochemical process in a carbon material capable of intercalating and withdrawing lithium in an ionized state. However, the uses mentioned for the lithium ion capacitor are power storage elements for railways, construction machinery and automobiles, and such uses require even greater improvement in the charge/discharge cycle characteristic under high load.
With charge/discharge of a nonaqueous lithium power storage element, the negative electrode active material layer undergoes repeated intercalation and release of lithium ions and expansion and shrinkage, and the stress causes the negative electrode active material layer to detach from the negative electrode power collector, lowering the high load charge/discharge cycle characteristic. Methods for minimizing such detachment of the negative electrode active material layer include methods of modifying the type and amount of binder.
However, since binders with high binding capacity have a low swelling property for nonaqueous electrolytic solutions, and the increase in binder causes obstruction of the lithium ion diffusion channels in the nonaqueous electrolytic solution, the internal resistance increases, resulting in increased overvoltage during high load charge/discharge cycling, and significant increase in the coating film or deposits due to reductive decomposition of the nonaqueous electrolytic solution at the negative electrode active material layer, and making it difficult to obtain a satisfactory high load charge/discharge cycle characteristic.
PTL 5 proposes a lithium ion capacitor having low deviation in thickness of the electrode layer on the front and back sides. With the electrode of PTL 5, however, no consideration is given to minimizing increase in resistance during high load charge/discharge cycling at the lithium compound-containing positive electrode, or minimizing gas generation due to decomposition of the lithium compound under high voltage.